The present invention relates generally to thin film resistors and more specifically to a method of fabricating thin film resistors which are capable of laser trimming.
In many integrated circuits, the accuracy and matching of the resistors is very importance. This includes not only digital-to-analog converters, differential amplifiers and subscriber line interface circuits, but other integrated circuits. Heretofore, the method of adjusting resistors in these circuits included providing a resistive material and laser trimming the resistive material. Commonly used resistive materials which are capable of laser trimming have included nickel-chromium (nichrome) and tantalum nitride. These materials generally have a low resistivity and therefore require substantial amounts of surface area in order to produce the desired resistance and maintain laser trimmability of the resistor. Also, excess trimming of the resistor is needed to improve ratio matching, thus resulting in resistor instability. The commonly used laser is a YAG. Although other materials can be used, for example, chrome silicide or cermets which have higher sensitivity, than nichrome, a different laser must be used providing substantially higher power for trimming purposes. However, substantial laser trimming of these chrome silicide resistors causes resistor shift over life and therefore is detrimental to the remainder of the circuit. To solve this problem, several nickel chromium resistor segments are used to satisfy circuit resistance requirements.
Also contact materials connected to the ends of the resistors or interconnecting resistor segments have generally included aluminum. These interfaces deteriorate over a period of time. Similarly, the use of series connected resistors in order to obtain the required resistance greatly increased the number of contacts or interconnects of aluminum and therefore increase the amount of contact area which is subject to deterioration.
Thus, it is an object of the present invention to provide an improved thin film resistor taking up less surface area.
Another object of the present invention is to provide a laser trimmable thin film resistor which takes up less surface area.
An even further object of the present invention is to provide a laser trimmable thin film resistor with minimum contact degragation.
These and other objects are achieved by providing a resistor having a first portion of a high resistivity material and a second portion of a low resistivity material and laser trimming the low resistivity material portion. The use of the higher resistivity material greatly reduces the surface area required for the total resistance. This also reduces greatly the number of contacts or interconnects of aluminum and therefore the possibility of contact area deterioration. In an integrated circuit having two resistors which must be matched, one resistor, having total resistance lower than the second resistor, would include the laser trimmable portion of the lower resistivity material. Alternatively, both the resistors could be formed of substantially equal total resistance and each include a laser trimmable portion to improve accuracy and matching.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.